Antitachycardia therapy devices, such a antitachycardia pacemakers or antitachycardia defibrillators-cardioverters, utilize heart rate detection circuitry to determine the onset of a cardiac arrhythmia. Such rate detection circuitry typically includes discrete rate zones for the classification of arrhythmias. The rate zones, which are typically set through one or more user programmable parameters, dictate what type of device response, i.e., what type of antitachycardia therapy, will be provided whenever the detected heart rate falls within a given rate zone. For example, a user may define a low rate ventricular tachycardia zone which is identified by a low rate limit, rate A (e.g., 135 bpm), and a high rate limit, rate B (e.g., 175 bpm). Any detected heart rate that falls within this zone would thus be identified as a low rate ventricular tachycardia, and such identification would trigger an appropriate therapy from the antitachycardia therapy device. Similarly, a high rate tachycardia classification zone, e.g., between 175 and 240 bpm, may be defined that borders the low rate tachycardia zone. Thus, any detected heart rate that falls within the high rate tachycardia zone would be identified as a high rate ventricular tachycardia, thereby triggering another type of therapy from the antitachycardia therapy device. Further, in some devices, a third zone may be added just above the high rate tachycardia zone. Hence, in this fashion, the user can set up a single large rate zone or as may smaller bordering rate zones as desired, with each rate zone providing a particular one of a variety of independent antitachycardia therapies that is automatically applied by the device in order to terminate a perceived arrhythmia. As needed, the detection and classification of arrhythmias may be further based on additional parameters, in addition to heart rate, such as the hemodynamics identified by a particular sensor, and/or other rate parameters, such as the onset, stability or duration of the detected heart rate.
Unfortunately, the selection of discrete rate zones may prove to be problematic when a patient has a normal sinus rate that enters into one of the tachycardia zones during exercise or during other periods of physiological stress. Although various discriminating parameters, such as the onset and stability of the detected rate or rhythm, may be used in an attempt to minimize the detection of a fast normal sinus rate as a tachycardia rate, the fast rhythm or rate may still be incorrectly classified as an arrhythmia, resulting in antitachycardia therapy being provided by the device. Such antitachycardia therapy could be inappropriate, and even dangerous, because it may actually induce an actual arrhythmia when provided incorrectly.
Hence, it is apparent that there is a critical need for rate detection circuitry in an antitachyardia pacemaker or defibrillator-cardioverter that minimizes the risk of incorrectly identifying and classifying a fast sinus rate (for which antitachycardia therapy is not needed nor desired) as an arrhythmia (for which antitachycardia therapy is needed and desired).